Data coming from tape (or disk) is usually malformed due to a variety of effects, among them offset and rolloff. In the former, the data is not centered about its ideal values, and in the latter, there is insufficient energy at high frequencies. The result is usually a severe degradation in signal quality, whether measured as SINR (signal-to-noise ratio) or BER (bit error rate). By way of example, in a typical arrangement, an original signal, i.e. a high quality signal, is initially recorded onto a storage medium, such as disk, tape, optical or other digital data storage medium. Unfortunately, often only a suboptimal signal is obtainable from the storage medium. Accordingly, there is a need for a system and method for cleaning up the suboptimal signal so that it can match as nearly as possible the original signal.
Digital data recovery systems, e.g., disk, tape, optical, or other digital data recovery systems, are designed to address these deficiencies, with the ultimate output being a data stream, usually but not necessarily of three levels, which is properly centered and in which the high-frequency energy has been boosted. Previous attempts to address the above-noted problems involved the use of direct feedback for adaptive filter training, linear interpolation, and/or reverse interpolation.
Digital data recovery systems typically receive oversampled data, that is, data sampled at a rate 1/Ts that is slightly higher than the bit rate 1/T, to allow for speed variations in the source. A Fractionally-Spaced Equalizer, or “FSE”, which is a finite-impulse response filter, is generally used to equalize the oversampled data before it is delivered to a timing recovery unit. The timing recovery unit typically includes an interpolator, a slicer, a phase detector, a loop filter and an oscillator. This filter is often made adaptive, so that its coefficients adjust dynamically and continuously in response to changes in the input signal. In this case, the filter is called an Adaptive Fractionally-Spaced Equalizer, or “AFSE”. However, direct extraction of an error signal in the oversampled domain is not possible, so the AFSE must be trained using an error signal fed back from the bit rate domain following the timing recovery unit. Further, AFSEs are known to suffer from a phenomenon called “tap wandering” which can cause them to become unstable. Additionally, while algorithms or other leakage mechanisms have previously been utilized to treat the problem of tap wandering, the results are known to be suboptimal. Still further, the adaptive hardware required to operate a continuously updating FSE/AFSE greatly increases the complexity of the digital data recovery channel.